Process improvements to maximize retention, drainage, formation, and drying are continually being demanded. The introduction of closed loops have increased the complexity in these paper making systems. The desire to maximize all operating parameters simultaneously and the properties of the paper being made, via chemical additives, has proved troublesome.
Currently, many paper makers attempt to maximize filler and pulp fines retention by the addition of a high molecular weight, water soluble polymer, such as derivatized polyacrylamide. The derivatized polyacrylamide used may be cationic or anionic in nature. In general, it has been found that the higher the molecular weight of the material used, the greater has been the retention. On the other hand, as the molecular weight of the polyacrylamide is increased, sheet formation decreases. The same is true for increasing the amount of polyacrylamide used, namely retention increases, but sheet formation suffers.
Britt (Tappi (1980) 63, 5, 105-108) recognized that if fibrous flocs formed as a result of the addition of polymer are such that they are serious and undesirable, then overflocculation has occurred. Britt also noted, however, that whether a given type or amount of polymer results in overflocculation greatly depends upon the turbulence prevailing during and after the addition of the polymer. Pummer (Papier (1973) 27, 10417-422) had previously shown that polyacrylamides caused excessive agglomeration of the fines particles and thus lowered the optical qualities of the paper.
The understanding of the interrelationship between energy (such as turbulence or shear) to which the stock is subjected prior to sheet formation, as well as where the necessary additives are introduced, became the focus of increased attention. Luner and Keitaaniemi (Tappi Paper Makers Conference, (1984) 95-106) noted that for the polyacrylamide tested, first-pass retention increases, reaches a maximum, and then declines with increasing energy input.
Stratton (Tappi (1983) March 141-144) drew a similar conclusion and stated that a compromise is necessary because the polymer must be distributed uniformly prior to adsorption, but once adsorbed it is important to avoid extreme turbulence. The compromise suggested by Stratton was to introduce the polymer at the outlet of a high-shear element (e.g. fan pump or pressure screen) where turbulence is still adequate for polymer distribution but not so extreme as to reduce retention.
A series of papers addressed the subject of the shear associated with the various elements in the paper making process and their effect on retention systems. van de Ven and Mason (Tappi (1981) 64, 9, 171-175) concluded that the forces that predominate are the hydrodynamic forces rather than the colloidal forces.
Tam Doo et.al, (J. Pulp and paper Science (1983) July, J80-J88) estimated the fluid shear rates and maximum shear stress on fiber walls for various components of the paper making system and compared these values against those obtained on a standard piece of laboratory equipment (namely a Dynamic Drainage Jar). This comparison allowed a more realistic assessment of polymeric retention systems, with respect to simulating both type of polymer and point of addition.
The relationship between shear and retention was further investigated by Hubbe (Tappi (1986) August 116-117) who came to the conclusion, somewhat at odds with the teachings of Britt, Stratton and Mason, that polyacrylamide should be added prior to the fan pump to assure efficient mixing. Waech (Tappi Engineering Conference (1982); Tappi (1983), March 137-139), concurred with Hubbe and showed that the addition of the polymer ahead of the fan pump, when compared with the polymer added after the fan pump, gave similar retention and improved formation. These experiments were performed on a system in which all the filler was added after the polymer, a somewhat unrealistic model for actual paper making.
Booth (U.S. Pat. No. 2,368,635) was the first to utilize bentonite as a retention aid, proposing that the bentonite acted as both a coagulant of finely divided particles and an absorber of contaminating substances.
Pye (U.S. Pat. No. 3,052,595) utilized a combination of bentonite and anionic or neutral polyacrylamide to achieve much lower turbidity in the white water of a laboratory scale paper making device. The preferred method of addition was to add the bentonite prior to the polyacrylamide.
The use of bentonite was also investigated by Pummer (DE Patent 2262906) who claimed that the addition of aluminum sulphate and bentonite to the stock prior to the addition of polyethyleneimines, polyamide-polyamines or polyetheamines increased the performance of the polymers.
Auhorn (Wochenblatt Fur Papierfabrikation (1979) 13,493-502) also utilized bentonite as an additive, prior to the addition of polyethyleneimine, to reduce the amount of oxidizable substances in the paper and also to increase the effect of the polyethyleneimine that was subsequently added to the paper making stock.
Auhorn in later work (Wet End Paper Technology Symposium (1981) March, Munich), enlarged on his earlier work to include both polyethyleneimine and polyacrylamide. The conclusions on a lab scale were similar to the earlier work, although these improvements were never fully realized on a paper making machine trial.
Langley and Litchfield (U.S. Pat. No. 4,305,781) proposed a similar system utilizing a bentonite clay and a largely non-ionic, high molecular weight polymer to be used on cellulosic suspensions substantially free of filter. It is suggested that the bentonite is added to thick stock, to the hydropulper or to the re-circulating white-water. The polymer is ideally added after the last point of high shear, typically after the centri-screens and just before the head-box.
Bentonite-polymer systems were not and are not the only example of what are known as "multi-component or microparticulate retention systems."
As early as 1975 Arledter (Papier 29, 10a, 32-43) used a combination of polyethylene oxide and melamine formaldehyde resin to improve retention. Addition of the polyethylene oxide at either the machine chest or headbox gave comparable retentions.
Svending (U.S. Pat. No. 4,385,961 and U.S. Pat. No. 4,388,150) described a retention system that, to some extent, gave both increased retention and drainage without a concomitant decrease in formation. This retention system is comprised of cationic potato starch and a colloidal silicic acid. Little mention is made of where these addition points are relative to the various points of shear except that starch should be added to, and well mixed with, the stock prior to addition of the silica for the best results. This system has been marketed under the name Composil by ProComp, a joint venture of EKA and duPont, Marietta, GA. However, the usefulness of the system is limited because it is much less effective in an acid paper making system, and it is an expensive system because both the starch and silica costs are quite high, and significant amounts of both are required.
Anderson in W086/05826 describes modification of the surface of silica with aluminum ions to produce a colloidal silica particle that maintained its efficiency over the whole pH range utilized by paper makers, namely pH 4-8. This aluminum modified silicic acid solution was used in combination with a cationic polyacrylamide. Many examples of drainage and retention improvements are given using standard laboratory practices. In all examples given, the polymer was added prior to the aluminum modified silicic acid solution.
Two publications related to the improvement in sizing of paper (Finnish patents 67735 and 67736) used a combination of a cationic polymer retention agent including polyacrylamide, polyethyleneimine, polyamine, polyamideamine or melamine formaldehyde polymer and an anionic polymeric binding agent including colloidal silicic acid, bentonite, carboxymethylcellulose or anionic polyacrylamide. Although these papers focus on the effect on sizing, it is recognized that the retention of filler and fine particles also improves.
Finnish Patents 67735 and 67736 espouse the use of similar chemical additives, the difference being that in 67735 the size is applied to the already formed sheet of paper and in 67736 the size is applied to the water suspension prior to the formation of the sheet of paper. However, the definition of the retention mixture and method of application remain unchanged between the patents. Patent 67735 claims the cationic compounds can be present between 0.2 and 40 lbs./ton and the anionic compounds can be present between 0.2 and 12 lbs./ton.
Lorz (U.S. Pat. No. 4,749,444) suggests that procedures outlined by Langley in U.S. Pat. No. 4,305,781 and European Patent 0017353 and by Pye (U.S. Pat. No. 3,052,595) both suffer from the same defect, namely, over flocculation of the sheet. Lorz outlines a method of adding "bentonite" to the thick stock (consistency 2.5 to 5.0% by weight), followed by agitation and dilution to a thin stock (consistency 0.3 to 2% by weight), followed by addition of a cationic polyelectrolyte and, after thorough mixing, a high molecular weight (1 million to 20 million average molecular weight) anionic or cationically charged polymer is added. Although this process results in improved drainage, no values are given for formation. The examples given are filler-free stock suspensions.
This work by Lorz was an extension of the coagulation-flocculation theory that is now generally accepted. The residual charge on the furnish, as measured by a cationic demand, zeta potential, mobility or colloid titration procedure, should be close to zero to maximize the coagulation process. Effective coagulation results in small, very shear-sensitive, agglomerates but these small agglomerates can be flocculated by the use of high molecular weight polymers. This flocculation is often achieved by the use of cationic polyelectrolytes as described by Lorz. This results in acceptable flocculation parameters, and minimizes the use of the high molecular weight polymer, while maintaining sufficient retention and drainage.
Langley (Tappi (1986) Paper makers Conference) outlined another system utilizing a combination of bentonite and polyacrylamide, where an excess of high molecular weight linear synthetic cationic polymer is added to an aqueous cellulosic suspension before shearing the suspension, and adding bentonite after shearing and then draining the purified suspension. This system is an expensive system because (1) five times as much high molecular weight polymer was used in comparison with conventional polymeric retention aid use levels, and (2) there was the additional expense of the bentonite.
European application 0 373 306 discloses a retention aid composition comprising a water dispersible colloidal siliceous material in intimate association with a low molecular weight, water soluble, high charge density organic polymer, such as a polyacrylic acid or a polyamine, the ionicity of the siliceous material being significantly modified by the charge on the polymer. The composition is produced by reacting the siliceous material and the organic polymer in an aqueous phase system. The composition is said to be suitable for use as a retention/drainage agent in paper production, preferably after the addition of a conventional high molecular wieght flocculating agent.